(1) Field of the Invention
The present invention relates to a variable resistor to mount on a surface such as a printed circuit board, whose resistance is adjustable by rotating a slider on a resistive element formed on an insulating substrate.
(2) Description of the Prior Art
A known example of such a variable resistor is illustrated in FIGS. 1 and 2A through 2C. FIG. 1 is an exploded perspective view, FIG. 2A a plan view, FIG. 2B a cross sectional view along the A--A line of FIG. 2A, FIG. 2C a bottom view, and FIG. 2D a vertical cross sectional view illustrating an external electrode in details.
In the illustrated variable resistor, an insulating substrate 1 has a bore 1a at the substantial center thereof. The insulating substrate 1 includes an arcuate resistive film 2 formed on the upper surface thereof concentrically with the bore 1a. An external electrode 3 is electrically connected to one end of the resistive film 2, and another external electrode 4 to the other end thereof. As illustrated in FIG. 2D, the external electrode 3 comprises silver-palladium alloys 3a and 3b respectively printed and fired on the upper and bottom surfaces of the insulating substrate 1 and another silver-palladium alloy 3c formed by such a method as dipping and fired on the end surface thereof. The resistive film 2 is printed on the insulating substrate 1 thereafter. An electrode 5 comprises a metal plate, on which a hollow central cylinder 6 is integrally formed by such a method as drawing. The electrode 5 is fixed to the bottom and end surfaces of the insulating substrate 1 with the hollow central cylinder 6 inserted into the bore 1a. A slider 7 comprises a bore 7a at the substantial center thereof, a contact 7b for contacting the resistive film 2, and a cutout 7c. The contact 7b and the cutout 7c are on the outer periphery of the slider 7. A metallic rotor 8 has a bore 8a at its substantial center, a cross-shaped driver groove 8b on its upper surface, and a projection 8c at a peripheral position on the bottom surface thereof. The hollow central cylinder 6 of the electrode 5 extends through the bores 7a and 8a of the slider 7 and rotor 8, with the projection 8c engaged with the cutout 7c of the slider 7. The assembly of the slider 7 and rotor 8 are rotatably mounted on the insulating substrate 1 by caulking the top of the hollow central cylinder 6 around the mouth of the bore 8a. The slider 7 and rotor 8 are rotatable in unison by the engagement between the cutout 7c and the projection 8c. With this variable resistor, resistance is adjusted as follows: The tip of a driver is fitted into the driver groove 8b and is rotated, which causes the slider 7 and the rotor 8 to rotate, whereby the contact 7b slides on the resistive film 2 to adjust the resistance between the external electrode 3 or 4 and the electrode 5.
Another known variable resistor is illustrated in FIGS. 3A and 3B. This variable resistor is contained in a case 13 in order to realize the mounting by flow soldering. As for the same parts with those in FIGS. 1 and 2A through 2C, identical numbers are used and the explanation is omitted.
This variable resistor comprises electrodes 9, 10 and 11 formed of metal plates. The electrodes 9 and 10 are attached to the same end surface of an insulating substrate 1 and electrically connected to the respective end of the resistive film 2. The electrodes 9 and 10 correspond to the electrodes 3 and 4 of the foregoing variable resistor, the electrode 11 to the electrode 5 thereof, and a central cylinder 12 to the hollow central cylinder 6 thereof. The slider 7 and the rotor 8 are rotatably mounted on the insulating substrate 1 by caulking the top of the central cylinder 12 around the mouth of the bore 8a of the rotor 8. The resin case 13 has an opening on the upper surface thereof. The insulating substrate 1 is insert-molded into the resin case 13, with the electrodes 9, 10 and 11 exposed outside and with the upper surface of the substrate 1 forming the floor of the inner space. The exposed electrodes 9, 10 and 11 are bent over to the bottom surface of the case 13. A heat-resistant film covering 14 is sticked around the opening of the case 13 to seal the opening. The film covering 14 is formed of a transparent material, through which the interior of the variable resistor is seen as shown in FIG. 3A.
After this variable resistor is mounted on a printed circuit board or the like by such a method as flow soldering, the film covering 14 is broken with a driver or the like, and then the slider 7 and the rotor 8 are rotated with the driver to cause the contact 7b to slide on the resistive film 2 for resistance adjustment.
The above two known variable resistors have the following disadvantages.
In the variable resistor shown in FIGS. 1 and 2, the resistive film 2 and the slider 7 are exposed outside. Therefore, flow soldering, which provides high productivity, cannot be used for mounting it on a printed circuit board, but reflow soldering have to be instead. Moreover, even in mounting by reflow soldering, flux tends to be scattered onto the resistive film 2 and the slider 7 to cause imperfect contact.
This variable resistor employs the silver-palladium alloy for the external electrodes 3 and 4 to prevent silver from leaching in solder and to improve the abrasion-resistance against solder of the external electrodes 3 and 4. However, the above alloy is expensive and is not perfect in preventing silver from leaching in solder. Also, because the alloy 3c on the end surface of the insulating substrate 1 is inevitably thick, glass frit is exposed on its surface and makes it hard to solder, which necessitates complicated preliminary soldering.
The variable resistor shown in FIG. 3, which is contained in the case 13, can be mounted on a printed circuit board by flow soldering, but includes many parts and so is troublesome and costly to manufacture. Moreover, when breaking the film covering 14 with a driver or the like, fragments of the film covering 14 tend to be scattered inside and outside the case 13, thereby causing imperfect contact inside the variable resistor or bringing a detrimental effect to the device on which the variable resistor is mounted. Further, once the film covering 14 is broken, the resistive film 2 and the slider 7 are exposed. This means the device cannot be re-adjusted or re-washed.